The space shuttle Atlantis moved out to its seaside launch pad in Florida early Tuesday to prepare for NASA's long-delayed final flight to the iconic Hubble Space Telescope.

Moving slowly atop an Apollo-era carrier vehicle, Atlantis began the 3.4-mile (5.4-km) trip to Pad 39A at NASA's Kennedy Space Center at 3:54 a.m. EDT (0754 GMT) and reached the launch site about seven hours later.

With Atlantis at the launch pad, NASA will once more prepare the spacecraft for a May 12 launch to the Hubble Space Telescope. Five back-to-back spacewalks are planned during the 11-day mission to upgrade the orbital observatory and extend its life through at least 2013.

The mission has been delayed since last fall, when a data handling unit aboard Hubble unexpectedly failed just weeks before Atlantis was due to launch toward the space observatory.

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NASA engineers revived Hubble from the failure and began preparing a spare to launch aboard Atlantis and be added its overhaul mission.

The spare data handling unit arrived at the Kennedy Space Center on Monday, NASA spokesperson Allard Beutel told SPACE.com from the Cape Canaveral, Fla., spaceport.

Commanded by veteran spaceflyer Scott Altman, Atlantis's STS-125 mission to Hubble will send seven astronauts on the fifth and final service call to the orbiting laboratory.

The astronauts are expected to add a new camera, replace gyroscopes and batteries, as well attempt to repair equipment that was never designed to be fixed in space. They also plan to attach a docking ring to the telescope so a robotic vehicle could visit it in the future.

Tile repaired

Beutel told SPACE.com that aside from a minor ding to one of the heat-resistant tiles on Atlantis that required repair, the shuttle's preparations for launch have gone smoothly.

Last week, a shuttle worker accidentally dropped a torque wrench socket while tightening bolts on the bipod strut that connects the shuttle's nose with its 15-story external fuel tank. The socket bounced off a beam and dinged a tile near the fuel inlets on the belly of Atlantis.

"It was something that was very repairable," Beutel said. Engineers filed the small ding with heat-resistant putty and molded it to fit the shuttle's aerodynamic lines, he added.

Efforts to prepare a second shuttle, the Endeavour orbiter, to serve as a rescue ship for the Atlantis astronauts are also going well, Beutel said.

Because Hubble orbits the Earth higher and in a different inclination than the International Space Station, Atlantis would not be able to ferry its crew to the orbiting laboratory to seek refuge should the shuttle suffer critical damage. Instead, NASA is preparing Endeavour and a skeleton crew to be ready to lift off from a second launch pad to retrieve the Atlantis astronauts if needed.

Beutel said that Endeavour is on track to leave its hangar and move into NASA's cavernous Vehicle Assembly Building on April 10. The shuttle is then expected head out to the seaside Pad 39B launch site on April 17.

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